ci: Test the KHR-GL* CTS cases with softpipe.

I've been hacking on softpipe in the process of trying to delete a bunch
of core Mesa code, and want to make sure I don't regress desktop GL
either.  The run takes under a minute and a half.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6466>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 2765f39..47ca4bb 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -961,6 +961,15 @@
     DEQP_VER: gles31
   extends: softpipe-gles2
 
+# Note that KHR-GL3* test sets include all tests from the previous
+# version, so we only need to run one test list (unlike dEQP-GLES,
+# where the test sets are separate).
+softpipe-gl:
+  variables:
+    DEQP_VER: gl33
+  extends:
+    - softpipe-gles2
+
 virgl-gles2-on-gl:
   variables:
     DEQP_VER: gles2